feat(feed): use exponential backoff for broken feeds

Instead of having a hard limit of 3 (regular) tries for feeds with errors, use
an exponential backoff by default, with an upper-limit of one week.

See #3334 for context/discussions.

Author: jvoisin

internal/config/options.go

@@ -489,8 +489,8 @@ func NewConfigOptions() *configOptions {
 				},
 			},
 			"POLLING_PARSING_ERROR_LIMIT": {
-				ParsedIntValue: 3,
-				RawValue:       "3",
+				ParsedIntValue: 0,
+				RawValue:       "0",
 				ValueType:      intType,
 				Validator: func(rawValue string) error {
 					return validateGreaterOrEqualThan(rawValue, 0)

internal/config/options_parsing_test.go

@@ -1058,8 +1058,8 @@ func TestPollingLimitPerHostOptionParsing(t *testing.T) {
 func TestPollingParsingErrorLimitOptionParsing(t *testing.T) {
 	configParser := NewConfigParser()
 
-	if configParser.options.PollingParsingErrorLimit() != 3 {
-		t.Fatalf("Expected POLLING_PARSING_ERROR_LIMIT to be 3 by default")
+	if configParser.options.PollingParsingErrorLimit() != 0 {
+		t.Fatalf("Expected POLLING_PARSING_ERROR_LIMIT to be 0 by default")
 	}
 
 	if err := configParser.parseLines([]string{"POLLING_PARSING_ERROR_LIMIT=5"}); err != nil {

internal/model/feed.go

@@ -6,6 +6,7 @@ package model // import "miniflux.app/v2/internal/model"
 import (
 	"fmt"
 	"io"
+	"math"
 	"time"
 
 	"miniflux.app/v2/internal/config"
@@ -135,6 +136,16 @@ func (f *Feed) ScheduleNextCheck(weeklyCount int, refreshDelay time.Duration) ti
 	// Use the RSS TTL field, Retry-After, Cache-Control or Expires HTTP headers if defined.
 	interval = max(interval, refreshDelay)
 
+	// Use a binary exponential backoff for feeds with parsing errors.
+	// https://en.wikipedia.org/wiki/Exponential_backoff
+	if f.ParsingErrorCount > 0 {
+		backoff := time.Duration(math.Pow(2, float64(f.ParsingErrorCount))) * time.Hour
+		// Set a hard limit at one week
+		if backoff.Hours() < time.Hour.Hours()*24*7 {
+			interval += backoff
+		}
+	}
+
 	// Limit the max interval value for misconfigured feeds.
 	switch config.Opts.PollingScheduler() {
 	case SchedulerRoundRobin:

miniflux.1

@@ -504,7 +504,7 @@ The maximum number of parsing errors that the program will try before stopping p
 .br
 Once the limit is reached, the user must refresh the feed manually. Set to 0 for unlimited.
 .br
-Default is 3\&.
+Default is 0\&.
 .TP
 .B POLLING_SCHEDULER
 Determines the strategy used to schedule feed polling.